1. Field of the Invention
The present invention relates to an image forming apparatus for forming a toner image on a recording material. The above-mentioned image forming apparatus includes, for instance, a copying machine, a printer, a FAX (facsimile machine), and a multifunction apparatus equipped with those functions in plurality.
2. Description of the Related Art
Conventionally, the above-mentioned image forming apparatus generally employs as a fixing system for fixing an unfixed toner image on a recording material a thermal fixing system in view of safety and an excellent fixability. In the thermal fixing system, the unfixed toner images of the recording material is heated to be melted so as to be fixed on the recording material.
Also, in recent years, a heating apparatus of a belt heating type has been practically utilized in view of a quick start and energy savings. In other words, a pressure contact nip portion (hereinafter, referred to as “fixing nip portion”) is formed by sandwiching a heat resistance resin belt (hereinafter, referred as “fixing belt”) as an image heating member between a heating member, for example, a ceramics heater, and a pressure roller serving as a pressure member. Then, a recording material which carries an unfixed toner image formed thereon is introduced between the fixing belt and the pressure roller of the fixing nip portion, and the recording material is sandwiched and transported in combination with the fixing belt. As a result, the unfixed toner image is fixed on a surface of a recording material by being applied by pressure of the fixing nip portion while the heat of the ceramics heater is applied thereto via the fixing belt.
The heating apparatus of belt heating type can be brought into a heatable condition even if an operation of energizing the heater is commenced after an image forming apparatus has received a print signal until the recording material reaches the heating apparatus, while the energizing operation to the heater during standby status is not required. As a consequence, the above-mentioned heating apparatus of belt heating type may constitute such a heating fixing apparatus having a superior characteristic without wasting energy in view of energy savings.
Furthermore, another fixing system has been proposed in which a pressure member is arranged in such a manner that the pressure member is located via a belt opposite to a fixing roller.
Even in any of the cases where any of the above-mentioned fixing system is employed, when a recording material is subjected to fixing within a fixing area, a sheet passing area surface of the heating roller has a substantially uniform temperature distribution.
However, in such a case where small size recording materials having widths smaller than a width of a maximum width size recording material which can be passed through an apparatus are continuously subjected to fixation in a fixing area, a temperature of a non-sheet passing area surface of the heating roller is excessively increased. The excessive increase in temperature is caused by the following reason. That is, when the small size recording materials are continuously passed, heat is partially stored in the non-sheet passing area where the recording materials are not passed, because the heat cannot be deprived by the recording materials.
The phenomenon is referred to as an “end portion increased temperature” of the fixing apparatus, or a “non-sheet passing portion increased temperature” thereof. When the edge portion increased temperature of the fixing apparatus becomes high, the high temperature exceeds temperature rise limitations as to fixing member structural components and a pressure roller, which may cause these components to be damaged.
In order to prevent such a non-sheet passing portion increased temperature, conventionally, when small size recording materials are passed, a so-called “throughput down control operation” is carried out by which because a rotating speed of a fixing operation is reduced, a temperature rise of the non-sheet passing portion is suppressed. However, the throughput down control operation eventually lowers the productivity.
In order to prevent the reduction of the productivity, as described in Japanese Patent Application Laid-Open No. S60-136779 and Japanese Patent Application Laid-Open No. 2003-076209, the following structures are known. That is, while an air blowing fan is provided in a fixing apparatus, the air blowing fan blows wind to a heat roller and a pressure roller of a non-sheet passing portion. Thus, while the temperature rise of the non-sheet passing portion may be suppressed, both the suppression of the non-sheet passing portion increased temperature and the superior productivity may be achieved.
In the apparatus described in Japanese Patent Application Laid-Open No. S60-136779, a cooling fan arranged on the fixing apparatus blows cooling wind to the non-sheet passing portion side. Also, while such an element for detecting a temperature of a fixing area when a small size paper is passed is provided, an ON/OFF control operation for the air blowing fan is carried out based on a signal value of the temperature detecting element.
In the apparatus described in Japanese Patent Application Laid-Open No. 2003-076209, in such a case where the cooling fan blows the cooling wind to the non-sheet passing area side, because a length of an air blowing port in the width direction is adjusted based on a width of a recording material to be employed, the above-mentioned non-sheet passing portion increased temperature may be avoided even with respect to papers having different sizes from each other.
In the apparatus described in Japanese Patent Application Laid-Open No. S60-136779 and Japanese Patent Application Laid-Open No. 2003-076209, in order to improve lowering of the productivity caused by the throughput down control operation, the air blowing fans are employed. As a result, when the small size recording materials are continuously passed while the air blowing fans are operated, the fixing operations are carried out at the normal fixing speeds.
However, when a cooling area of a fixing member by an air blowing fan is different from a width of such a recording material which is actually passed, for example, in such a case where a B5R width is passed when a non-sheet passing portion of an A4R width is cooled, a surface of the fixing member outside the A4R edge is cooled by the air blowing fan. Nevertheless, such an area from the B5R edge to the A4R edge becomes the non-sheet passing portion, because the actual sheet passing portion is the B5R width. As a result, although the temperature as to the non-sheet passing portion is increased, the portion is not cooled by the air blowing fan. But also, because the air blowing fan is operated, there is no possibility that the throughput down control operation is carried out. As a consequence, the temperature as to the above-mentioned area of the fixing member surface is excessively increased. Therefore, there is a risk that the fixing apparatus may be finally destroyed.
Among current copying machines, there are many such copying machines that detecting members for automatically recognizing sizes of recording materials are not equipped on main bodies thereof due to cost down purposes, namely, users set these sizes of recording materials which are used in these copying machines. In other words, the above-mentioned event may occur in the case where width sizes of recording material which are set/instructed by users are different from width sizes of recording materials which are actually used.
In such an apparatus equipped with an arrangement capable of preventing a non-sheet passing portion increased temperature by employing a cooling means such as an air blowing fan, in which a detecting member for automatically recognizing a recording material size is not provided, a safety characteristic capable of preventing destruction of the apparatus must be secured, while the apparatus destruction is caused by that a set size of the recording material is not coincident with an actual sheet passing size.
In the apparatus disclosed in Japanese Patent Application Laid-Open No. S60-136779, the ON/OFF operations of the air blowing fan are carried out based on the signal value of the element which detects the temperature of the fixing area, so the cooling operation for the fixing apparatus is carried out by the air blowing fan. However, the above-mentioned temperature detecting element is employed in the ON/OFF operations for the air blowing fan when a recording material having such a size that the air blowing fan is operated is passed, whereas the temperature detecting element is not so arranged in order that the temperature of the temperature raised portion can be detected when the air blowing area is not coincident with the sheet passing area. As a result, such a safety characteristic of the fixing apparatus when the user erroneously sets the recording material cannot be secured.
Also, as explained above, in such a case where the recording material is erroneously set, because the actual image size is different from the recording material size, there are some possibilities that such images of which edge portions are dropped out are discharged, and therefore, it is desirable to immediately stop printing operations.
Also, as described above, even when the air blowing area is not coincident with the sheet passing size, it is desirable for users that the image forming operation be accomplished without stopping the printing operation.
Further, in the apparatus described in Japanese Patent Application Laid-Open No. 2003-076209, the following technical idea is disclosed. That is, the different recording material sizes can be accepted by the cooling operation by the air blowing fan and by the opening width adjusting mechanism. However, the employment of such an opening width adjusting mechanism may cause a cost-up problem, and a cumbersome and expensive apparatus is necessarily required in order to accept a plurality of recording material sizes up to a minimum width paper such as a postcard.
Under such a circumstance, many apparatus are not equipped with the above-mentioned opening width adjusting mechanism. Normally, these apparatus cannot accept postcard sizes of which use frequencies are low, but are equipped with a fixed opening, which is fitted to such a recording material size of which use frequency is the highest value.
Even such the apparatus equipped with fixed opening widths has a similar problem to that of the fixing apparatus equipped with the opening width adjusting mechanism, and therefore, the safety characteristic when the recording material is erroneously set must be secured, and also, the image forming operation must be accomplished.